The simplest MOB devices are called proximity systems. The components include a receiver at the helm, which monitors the signal from a small transmitter, or fob, that can be worn by everyone on board. When the signal from a fob is interrupted — by immersion in the water, for instance, or by a distance beyond the effective radius (usually about 40 feet) — it triggers an alarm, marks a chart plotter and may shut down your engine if under power.
Proximity alarms are relatively inexpensive and reliable for a number of recreational situations. However, since the signal can be broken when batteries are low or compromised by line of sight, this type of device can be prone to false alarms. In addition, once the signal is broken, it doesn't return. As a result, those left onboard get a warning that someone has gone over, but it's up to them to locate the victim and make the rescue. This may not be problematic for the crew of a smaller, open boat with great visibility and maneuverability.
Raymarine's LifeTag ($695 with two fobs) is another proximity system. It functions as a stand-alone, but it can be integrated with Raymarine's proprietary SeaTalk network to sound an alarm, mark the MOB position on the plotter and program the autopilot to return to the place where the person went into the water. The system's 30-foot functional radius can be boosted with a second receiver ($469). Each receiver will cover up to 16 transmitters/fobs. Additional fobs are $115.
The MOBi-lert 720i ($899 with two transmitters) by Mobilarm is also a proximity system, but it can be linked through NMEA 0183 to a wide range of compatible GPS and chart plotters. As befits its higher price, it includes a color-coded fob ID scheme, so the skipper knows which fob (or person) has gone overboard. There are optional alarm and engine-cutoff configurations as well.
Another type of MOB technology is equipment that doesn't transmit until it's submerged. These systems boast longer ranges (up to a mile). Plus, the signals can be tracked because they transmit continuously after activation. The Virtual Lifeline by Maritech is one such service. Once the transmitter has been submerged, the engine is shut off. (Instant restarting is enabled with a rocker switch at the helm.) The system can be configured to provide any combination of alarm, GPS marking and/or engine kill function. Virtual Lifeline is available for any propulsion system. The price is $549 for a single-engine boat with two sensors. Fobs are $139 for two, and they must be serviced every two years.
A distinct advantage of water-activated systems is the continuous signal upon immersion, which allows for better tracking. "Let's say a person goes over in a 5-knot current," says Page Read, president of Emerald Industries, which makes a safety device called the Alert2. "Unless you have the ability to track, by the time you get back to where the alarm went off, the person may not be there anymore."
The Alert2 is a rugged component system with a receiver and antenna ($499), transmitters ($239 each) and a handheld radio direction finder ($799). It can be integrated with navigational software from Nobeltec and The Capn. Once activated, the signal can be tracked with the radio direction finder to locate the victim when strong currents or harsh conditions complicate visibility.
When using a water-activated transmission system, it's important for each passenger to wear the transmitter high on the body (usually near the chest). This way, the signal can reach the receiver.
This next class of MOB systems is highly sophisticated. The units are not necessary when you're trying to pull a pet back into the boat in a calm lake; rather, they are for full-blown search-and-rescue missions at sea. I'm referring to personal rescue beacons (PRB), which send signals (through manual activation) to the international search-and-rescue satellite system operated by Cospas-Sarsat on 406 MHz.
The McMurdo FastFind Max-G with GPS ($560) transmits a unique identification signal along with your current position. Unlike the EPIRB aboard your boat, it is registered to an individual user, rather than a vessel.
ACR's ResQFix ($600) is a 10-ounce PRB that sends a GPS position along with the rescue signal, broadcast on both 406 MHz and 121.5 MHz. The Vecta3 radio direction finder ($2,000) is a handheld locator antenna that reads all distress signal frequencies. When used with the ResQFix, an at-sea search is conducted until the helicopters and C-130s arrive. It's that good, although hopefully you'll never learn that firsthand.
As of February 2009, the 406 MHz frequency will be the only one monitored by the international search and rescue network, Cospas-Sarsat, which will phase out 121.5 MHz monitoring. This means the old workhorse Mini B by ACR, which transmits 121.5 MHz, is no longer in SAR duty. It certainly has earned its place in many ditch bags over the years and has saved a lot of lives, but its time has passed. Fortunately, the water-activated Mini-B300 ILS ($350) now has a place in ACR's MOB stable. As part of a closed system in combination with ACR's Vecta3 RDF, this little transmitter becomes a top-notch trackable rescue beacon when worn by each crew member.
Reproduced courtesy MotorBoating magazine and Glenn Law
You can read more about MOB devices and trialing in my ebook 'Voyage of the Little Ship 'Tere Moana' downloadable from my website http://www.sailboat2adventure.com/